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3-phase condo panel...rating?


Richard Moore
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Well...this is new for me. My first residential 3-phase panel. Really a "sub" panel in a 1962 apartment fed from a 100-amp 3-phase service disconnect in the meter room. Nothing more than 2-pole breakers, of course, within the unit's panel.

I'm going to report this as a 100-amp 3-phase 120-208 volt service even though, in reality, there is more than the usual 100-amps available. Would you describe it that way?

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I also had a couple of double-tapped breakers, but this was my "favorite". Check out the connection of the blue wire. Almost got it!

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Perhaps get out your multimeter and check the voltages. If you find the voltage to be 208V from one leg to neutral but 120V from the other two legs to neutral then it's an 'unbalanced neutral' configuration which is most popular among residences served by three phase power. A few installations might have a 'balanced' neutral configuration in which case all three lines will measure 120V from neutral. The balanced config has 208V between any two of the three lines which most appliance nameplates accept in lieu of 240V.

Marc

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How many of you recommend automatic evaluation of the system by a licensed electrician when you run across 3 phase residential systems? I've seen a few in the past 8 years and each time found a few of the common electrical issues. Since we rarely see 3 phase electrical systems during our inspections and little of our ongoing training seems to ever deal with 3 phase systems, I always recommend complete review and explain to the client why I recommend it.

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Original quote by Mr. Moore:

I'm going to report this as a 100-amp 3-phase 120-208 volt service even though, in reality, there is more than the usual 100-amps available. Would you describe it that way?

And by the way, 100 amps is 100 amps. As Freud once said, sometimes a cigar is just a cigar.

Marc

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I have actually done quite a bit of wiring of 3-phase panels, but that was back in my Scuba store days, many years and a couple of careers ago. I managed not to kill myself or anyone else. This particular panel didn't really seem that much different than a single-phase except for the extra bus-bar. The arrangement was different but not hard to understand.

I agree, sort of, that 100-amps is 100-amps, and it was the way I reported it...BUT...you can't convince me this doesn't have the potential for more load than a 100-amp single phase. For instance, if I had 3 50-amp (208-240v) loads, I could (I think) arrange three 50-amp double pole breakers to handle all those loads simultaneously, and each leg would end up with 100-amps. I could only use two of those in a single-phase 100-amp panel. I'm sure I am missing something, what is it, other than a solid grasp of electrical theory? [:-dunce]

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I think of it as getting 100A 'from more sources than a single phase'... (from more points on the 'rotating machine')... i.e., tapping similar current, but more 'lines' coming in from the device creating the juice.. (the 'rotating machine'). Electrical stuff gets me dizzy too sometimes.. The rotating machine is the usual source of juice (so far anyway) and that is a turbine somewhere..moved either by steam or water power.. I just say it's a '100A three phase service'.... this is actually has more oomph than single phase... kind of a like 3-man bicycle or something...

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  • 4 weeks later...

Well...this is new for me. My first residential 3-phase panel. Really a "sub" panel in a 1962 apartment fed from a 100-amp 3-phase service disconnect in the meter room. Nothing more than 2-pole breakers, of course, within the unit's panel.

I'm going to report this as a 100-amp 3-phase 120-208 volt service even though, in reality, there is more than the usual 100-amps available. Would you describe it that way?

Click to Enlarge
tn_200911252262_0014.jpg

40.38 KB

Click to Enlarge
tn_2009112522630_0028.jpg

99.86 KB

Click to Enlarge
tn_2009112522654_0023.jpg

52.36 KB

I also had a couple of double-tapped breakers, but this was my "favorite". Check out the connection of the blue wire. Almost got it!

Click to Enlarge
tn_200911252279_0026.jpg

40.73 KB

The service is in the meter room. The size of it is based on the main disconnect rating/service entrance conductor ampacity. The panel in the unit is a subpanel. Its ampacity is based on the size of the feeder breaker/conductor ampacity.

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The service is in the meter room. The size of it is based on the main disconnect rating/service entrance conductor ampacity. The panel in the unit is a subpanel. Its ampacity is based on the size of the feeder breaker/conductor ampacity.

I have no idea why you posted this very basic information, especially a month later, but let's get it right. The service equipment and service disconnect is in the meter room (first photo). The service size to the unit was what I was describing.

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The service is in the meter room. The size of it is based on the main disconnect rating/service entrance conductor ampacity. The panel in the unit is a subpanel. Its ampacity is based on the size of the feeder breaker/conductor ampacity.

I have no idea why you posted this very basic information, especially a month later, but let's get it right. The service equipment and service disconnect is in the meter room (first photo). The service size to the unit was what I was describing.

Sorry for the late post. I just joined. If nobody is allowed to comment on older threads, maybe it should be posted somewhere.

My apologies and Merry Christmas!!

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